1. Field of the Invention
The present invention relates to a surface acoustic wave device and a communication apparatus.
2. Description of the Related Art
As a conventional surface acoustic wave device, an inter-digital transducer structure (hereinafter referred to as IDT structure) as shown in FIG. 14 has been known. In the IDT structure device, electrodes 71 to 77 including signal input terminals 71 and 72 and signal output terminals 73 and 74 formed in a comb line shape are disposed between reflectors 78 and 79. In addition, a structure, where a serial resonator is connected to such an IDT structure surface acoustic wave device, is well known (see Japanese Patent Laid-Open Application No. 7-30367).
Such a surface acoustic wave device has been used as a filter of a mobile communication apparatus. Input terminals and output terminals of such a device are connected in an unbalanced state. However, in recent years, there are needs to use either or both input terminals and output terminals in a balanced state so as to directly connect such a filter to a peripheral circuit (for example, a mixer) disposed downstream thereof.
FIG. 15 is a schematic diagram showing the electrode structure of a surface acoustic wave filter having a serial resonator.
As shown in FIG. 15, the surface acoustic wave filter has balanced output terminals 53 and 54. The balanced output terminals 53 and 54 are directly connected to leads of output IDTs 55, 57, and 59. The balanced output terminals 53 and 54 are also connected to for example a balanced type mixer (not shown) disposed downstream of the balanced output terminals 53 and 54. Input IDTs 56 and 58 are disposed among the output IDTs 55, 57, and 59. The input IDTs 56 and 58 are connected to a low noise amplifier (LNA) disposed upstream thereof through a serial resonator 60. Reflectors 70 are disposed outside the IDTs 55 to 59. In addition, reflectors 70 are disposed outside the serial resonator 60. In FIG. 15, L represents an aperture length. The aperture length L is the width of each transducer. Since the serial resonator 60, located at the input side of the surface acoustic wave filter, widens the band width of a signal, the serial resonator 60 may be omitted depending on the band width of the signal.
When the input side of the filter is unbalanced and the output side of the filter is balanced, although an overvoltage at the unbalanced input terminals 51 and 52 is blocked by the serial resonator 60, an overvoltage at the balanced output terminals 53 and 54 is directly applied to the IDTs 55, 57, and 59.
If a part of the IDTs 55, 57, and 59 short-circuits due to discharging, a surge voltage, or the like, the performance of the filter deteriorates.
When the output side of the filter is balanced, the amplitude of an output signal of the balanced output terminal 53 in the case that the balanced output terminal 54 is terminated should be almost the same as the amplitude of an output signal of the balanced output terminal 54 in the case that the balanced output terminal 53 is terminated. In addition, the phase of the output signal of the balanced output terminal 53 in the case that the balanced output terminal 54 is terminated should be different by around 180xc2x0 from the phase of the output signal of the balanced output terminal 54 in the case that the balanced output terminal 53 is terminated. These conditions are required to obtain a good balanced output signal.
However, depending on the number of electrodes (or the number of pairs) of the IDT 55 to 59, the aperture length L thereof, and the polarities (directionality) thereof, an output signal of the balanced output terminal 54 may slightly differ from an output signal of the balanced output terminal 53. Thus, it is difficult to adjust such factors for a good balanced output signal.
FIG. 16 shows another electrode structure of a surface acoustic wave device used as a filter. In the structure shown in FIG. 16, output IDTs 62 and 64 are disposed among input IDTs 61, 63, and 65. However, the structure shown in FIG. 16 is the same as that shown in FIG. 15 except that the connecting method of leads of individual electrodes shown in FIG. 16 is different from that shown in FIG. 15. Thus, the same problem takes place against an overvoltage in the inverse direction.
Thus, in the conventional electrode structures, when an overvoltage in the inverse direction of a normal signal or noise is applied, the withstand voltage of the filter deteriorates.
In addition, it is difficult to satisfy conditions for a good balanced output signal and a good balanced input signal.
The present invention is made from the above-described point of view.
A first object of the present invention is to provide a surface acoustic wave device and a communication apparatus with an electrode structure that suppresses the withstand voltage from deteriorating against an overvoltage applied to a balanced terminal such as a balanced output terminal or a balanced input terminal.
A second object of the present invention is to provide a surface acoustic wave device and a communication apparatus that satisfy better balanced output conditions and better balanced input conditions.
To accomplish such objects, a first aspect of the present invention is a surface acoustic wave device, comprising an unbalanced input terminal to which a signal is added in an unbalanced state, an input transducer unit for exciting a surface acoustic wave with the electrical input added signal from the unbalanced input terminal, an output transducer unit for receiving the surface acoustic wave excited by the input transducer unit and obtaining a desired signal, a first balanced output terminal and a second balanced output terminal for outputting the desired signal obtained by the output transducer unit in a balanced state, a first resonator disposed between the first balanced output terminal and the output transducer unit, and a second resonator disposed between the second balanced output terminal and the output transducer unit.
A second aspect of the present invention is a surface acoustic wave device, comprising a first balanced input terminal and a second balanced input terminal to which a signal is added in a balanced state, an input transducer unit for exciting a surface acoustic wave with the signal added from said first balanced input terminal and said second balanced input terminal, an output transducer unit for receiving the surface acoustic wave excited by said input transducer unit and obtaining a desired signal, an unbalanced output terminal for outputting the desired signal obtained by said output transducer unit in an unbalanced state, a first resonator disposed between said first balanced input terminal and said input transducer unit, and a second resonator disposed between said second balanced input terminal and said input transducer unit.
A third aspect of the present invention is a surface acoustic wave device, comprising a balanced input terminal to which a signal is added in a balanced state, an input transducer unit for exciting a surface acoustic wave with the signal added from said balanced input terminal, An output transducer unit for receiving the surface acoustic wave excited by said input transducer unit and obtaining a desired signal, a first balanced output terminal and a second balanced output terminal for outputting the desired signal obtained by said output transducer unit in a balanced state, a first resonator disposed between said first balanced output terminal and said output transducer unit; and a second resonator disposed between said second balanced output terminal and said output transducer unit.
A fourth aspect of the present invention is a surface acoustic wave device, comprising an unbalanced input terminal to which a signal is added in an unbalanced state, a balanced output terminal from which a signal is put out in a balanced state, a lattice structured resonator group, connected between said unbalanced input terminal and said balanced output terminal, said lattice structured resonator group having a first output point and a second output point connected to said balanced output terminal, said lattice structured resonator group transducing the signal in the unbalanced state added from said unbalanced input terminal into a surface acoustic wave and an electric signal and outputting the surface acoustic wave and the electric signal to the first output point and the second output point, a first resonator disposed between the first output point and said balanced output terminal, and a second resonator disposed between the second output point and said balanced output terminal.
A fifth aspect of the present invention is a surface acoustic wave device, comprising a balanced input terminal to which a signal is added in a balanced state, an unbalanced output terminal from which a signal is put out in an unbalanced state, a lattice structured resonator group, connected between said balanced input terminal and said unbalanced output terminal, said lattice structured resonator group having a first input point and a second input point connected to said balanced input terminal, said lattice structured resonator group transducing the input signal in the balanced state added from said balanced input terminal through the first input point and the second input point into a surface acoustic wave and an electric signal and outputting the surface electric wave and the electric signal to said unbalanced output terminal, a first resonator disposed between the first input point and said balanced input terminal, and a second resonator disposed between the second input point and said balanced input terminal.
A sixth aspect of the present invention is a surface acoustic wave device, comprising a balanced input terminal to which a signal is added in a balanced state, a balanced output terminal from which a signal is put out in a balanced state, a lattice structured resonator group, connected between said balanced input terminal and said balanced output terminal, said lattice structured resonator group having a first output point and a second output point connected to said balanced output terminal, said lattice structured resonator group transducing the input signal in the balanced state added from said balanced input terminal into a surface acoustic wave and an electric signal and outputting the surface electric wave and the electric signal to the first output point and the second output point, a first resonator disposed between the first output point and said balanced output terminal, and a second resonator disposed between the second output point and said balanced output terminal.
A seventh aspect of the present invention is a communication apparatus, comprising first signal outputting means, second signal outputting means, and a surface acoustic wave device disposed between said first signal outputting means and said second signal outputting means, for filtering a signal, wherein said surface acoustic wave device has an unbalanced input terminal to which a signal is added in an unbalanced state, an input transducer unit for exciting a surface acoustic wave with the signal added from said unbalanced input terminal, an output transducer unit for receiving the surface acoustic wave excited by said input transducer unit and obtaining a desired signal, a first balanced output terminal and a second balanced output terminal for outputting the desired signal obtained by said output transducer unit in a balanced state, a first resonator disposed between said first balanced output terminal and said output transducer unit, and a second resonator disposed between said second balanced output terminal and said output transducer unit.
An eighth aspect of the present invention is a communication apparatus, comprising, first signal outputting means, second signal outputting means, and a surface acoustic wave device disposed between said first signal outputting means and said second signal outputting means, for filtering a signal, wherein said surface acoustic wave device hasa first balanced input terminal and a second balanced input terminal to which a signal is added in a balanced state, an input transducer unit for exciting a surface acoustic wave with the signal added from said first balanced input terminal and said second balanced input terminal, an output transducer unit for receiving the surface acoustic wave excited by said input transducer unit and obtaining a desired signal, an unbalanced output terminal for outputting the desired signal obtained by said output transducer unit in an unbalanced state, a first resonator disposed between said first balanced input terminal and said output transducer unit, and a second resonator disposed between said second balanced input terminal and said output transducer unit.
A ninth aspect of the present invention is a communication apparatus, comprising first signal outputting means, second signal outputting means, and a surface acoustic wave device disposed between said first signal outputting means and said second signal outputting means, for filtering a signal, wherein said surface acoustic wave device hasa balanced input terminal to which a signal is added in a balanced state, an input transducer unit for exciting a surface acoustic wave with the signal added from said balanced input terminal, an output transducer unit for receiving the surface acoustic wave excited by said input transducer unit and obtaining a desired signal, a first balanced output terminal and a second balanced output terminal for outputting the desired signal obtained by said output transducer unit in a balanced state, a first resonator disposed between said first balanced output terminal and said output transducer unit, and a second resonator disposed between said second balanced output terminal and said output transducer unit.
A tenth aspect of the present invention is a communication apparatus, comprising first signal outputting means, second signal outputting means, and a surface acoustic wave device disposed between said first signal outputting means and said second signal outputting means, for filtering a signal, wherein said surface acoustic wave device has an unbalanced input terminal to which a signal is added in an unbalanced state, a balanced output terminal from which a signal is put out in a balanced state, a lattice structured resonator group, connected between said unbalanced input terminal and said balanced output terminal, said lattice structured resonator group having a first output point and a second output point connected to said balanced output terminal, said lattice structured resonator group transducing the input signal in the unbalanced state added from said unbalanced input terminal into a surface acoustic wave and an electric signal and outputting the surface electric wave and the electric signal to the first output point and the second output point, a first resonator disposed between the first output point and said balanced output terminal, and a second resonator disposed between the second output point and said balanced output terminal.
An eleventh aspect of the present invention is a communication apparatus, comprising first signal outputting means, second signal outputting means, and a surface acoustic wave device disposed between said first signal outputting means and said second signal outputting means, for filtering a signal, wherein said surface acoustic wave device has a balanced input terminal to which a signal is added in a balanced state, an unbalanced output terminal from which a signal is put out in an unbalanced state, a lattice structured resonator group, connected between said balanced input terminal and said unbalanced output terminal, said lattice structured resonator group having a first input point and a second input point connected to said balanced input terminal, said lattice structured resonator group transducing the input signal in the balanced state added from said balanced input terminal through the first input point and the second input point into a surface acoustic wave and an electric signal and outputting the surface electric wave and the electric signal to said unbalanced output terminal, a first resonator disposed between the first input point and said balanced input terminal, and a second resonator disposed between the second input point and said balanced input terminal.
A twelfth aspect of the present invention is a communication apparatus, comprising first signal outputting means, second signal outputting means, and a surface acoustic wave device disposed between said first signal outputting means and said second signal outputting means, for filtering a signal, wherein said surface acoustic wave device has a balanced input terminal to which a signal is added in a balanced state, a balanced output terminal from which a signal is put out in a balanced state, a lattice structured resonator group, connected between said balanced input terminal and said balanced output terminal, said lattice structured resonator group having a first output point and a second output point connected to said balanced output terminal, said lattice structured resonator group transducing the input signal in the balanced state added from said balanced input terminal into a surface acoustic wave and an electric signal and outputting the surface electric wave and the electric signal to the first output point and the second output point, a first resonator disposed between the first output point and said balanced output terminal, and a second resonator disposed between the second output point and said balanced output terminal.
A thirteenth aspect of the present invention is a communication apparatus, comprising a reception amplifier for amplifying a signal received from an antenna, a band pass filter for extracting a desired band signal from the signal amplified by said reception amplifier, and a mixer for mixing the desired band signal extracted by said band pass filter and a local oscillation signal added from a PLL(Phase Locked loop) oscillator through a local oscillation filter, wherein said band pass filter has a balanced input terminal to which a signal received from said reception amplifier is added in an unbalanced state, an input transducer unit for exciting a surface acoustic wave with the signal added from said unbalanced input terminal, an output transducer unit for receiving the surface acoustic wave excited by said input transducer unit and obtaining a desired signal, a first balanced output terminal and a second balanced output terminal for outputting the desired signal obtained by said output transducer unit in a balanced state to said mixer, a first resonator disposed between said first balanced output terminal and said output transducer unit, and a second resonator disposed between said second balanced output terminal and said output transducer unit.
A fourteenth aspect of the present invention is a communication apparatus, comprising a reception amplifier for amplifying a signal received from an antenna, a band pass filter for extracting a desired band signal from the signal amplified by said reception amplifier; and a mixer for mixing the desired band signal extracted by said band pass filter and a local oscillation signal added from a PLL oscillator through a local oscillation filter, wherein said band pass filter has a balanced input terminal to which a signal received from said reception amplifier is added in a balanced state, an input transducer unit for exciting a surface acoustic wave with the signal added from said balanced input terminal, an output transducer unit for receiving the surface acoustic wave excited by said input transducer unit and obtaining a desired signal, a first balanced output terminal and a second balanced output terminal for outputting the desired signal obtained by said output transducer unit in a balanced state to said mixer, a first resonator disposed between said first balanced output terminal and said output transducer unit, and a second resonator disposed between said second balanced output terminal and said output transducer unit.
A fifteenth aspect of the present invention is a communication apparatus, comprising a reception amplifier for amplifying a signal received from an antenna, a band pass filter for extracting a desired band signal from the signal amplified by said reception amplifier, and a mixer for mixing the desired band signal extracted by said band pass filter and a local oscillation signal added from a PLL oscillator through a local oscillation filter, wherein said band pass filter has an unbalanced input terminal to which a signal received from said reception amplifier is added in an unbalanced state, a balanced output terminal from which a signal is put out in a balanced state, a lattice structured resonator group, connected between said unbalanced input terminal and said balanced output terminal, said lattice structured resonator group having a first output point and a second output point connected to said balanced output terminal, said lattice structured resonator group transducing the input signal in the unbalanced state added from said unbalanced input terminal into a surface acoustic wave and an electric signal and outputting the surface electric wave and the electric signal to the first output point and the second output point, a first resonator disposed between the first output point and said balanced output terminal, and a second resonator disposed between the second output point and said balanced output terminal.
A sixteenth aspect of the present invention is a communication apparatus, comprising a reception amplifier for amplifying a signal received from an antenna, a band pass filter for extracting a desired band signal from the signal amplified by said reception amplifier, and a mixer for mixing the desired band signal extracted by said band pass filter and a local oscillation signal added from a PLL oscillator through a local oscillation filter, wherein said band pass filter has a balanced input terminal to which a signal received from said reception amplifier is added in a balanced state, a balanced output terminal from which a signal is put out in a balanced state, a lattice structured resonator group, connected between said balanced input terminal and said balanced output terminal, said lattice structured resonator group having a first output point and a second output point connected to said balanced output terminal, said lattice structured resonator group transducing the input signal in the balanced state added from said balanced input terminal into a surface acoustic wave and an electric signal and outputting the surface electric wave and the electric signal to the first output point and the second output point, a first resonator disposed between the first output point and said balanced output terminal, and a second resonator disposed between the second output point and said balanced output terminal.
A seventeenth aspect of the present invention is a communication apparatus, comprising an FM modulator for FM-modulating an audio signal added from a microphone, a mixer for mixing the FM-modulated signal put out from said FM modulator and a local oscillation signal added from a PLL oscillator, a band pass filter for filtering a signal put out from said mixer, a transmission amplifier for amplifying a signal filtered by said band pass filter, and an antenna for transmitting a radio frequency wave that is a signal amplified by said transmission amplifier, wherein said band pass filter has a first balanced input terminal and a second balanced input terminal to which the signal mixed by said mixer is added in a balanced state, an input transducer unit for exciting a surface acoustic wave with a signal added from said first balanced input terminal, an output transducer unit for receiving the surface acoustic wave excited by said input transducer unit and obtaining a desired signal, an unbalanced output terminal which puts out the desired signal obtained by said output transducer unit to said transmission amplifier in an unbalanced state, a first resonator disposed between said first balanced input terminal and said input transducer unit, and a second resonator disposed between said second balanced input terminal and said input transducer unit.
An eighteenth aspect of the present invention is a communication apparatus, comprising an FM modulator for FM-modulating an audio signal added from a microphone, a mixer for mixing the FM-modulated signal put out from said FM modulator and a local oscillation signal added from a PLL oscillator, a band pass filter for filtering a signal put out from said mixer, a transmission amplifier for amplifying a signal filtered by said band pass filter, and an antenna for transmitting a radio frequency wave that is a signal amplified by said transmission amplifier, wherein said band pass filter has a balanced input terminal for inputting a signal mixed by said mixer in a balanced state, an input transducer unit for exciting a surface acoustic wave with a signal added from said balanced input terminal, an output transducer unit for receiving the surface acoustic wave excited by said input transducer unit and obtaining a desired signal, a first balanced output terminal and a second balanced output terminal for outputting the desired signal obtained by said output transducer unit to said transmission amplifier in a balanced state, a first resonator disposed between said first balanced output terminal and said output transducer unit, and a second resonator disposed between said second balanced output terminal and said output transducer unit.
A nineteenth aspect of the present invention is a communication apparatus, comprising an FM modulator for FM-modulating an audio signal added from a microphone, a mixer for mixing the FM-modulated signal put out from said FM modulator and a local oscillation signal added from a PLL oscillator, a band pass filter for filtering a signal put out from said mixer, a transmission amplifier for amplifying a signal filtered by said band pass filter, and an antenna for transmitting a radio frequency wave that is a signal amplified by said transmission amplifier, wherein said band pass filter has a balanced input terminal to which a signal mixed by said mixer is added in a balanced state, an unbalanced output terminal from which a signal is put out in an unbalanced state, a lattice structured resonator group, connected between said balanced input terminal and said unbalanced output terminal, said lattice structured resonator group having a first input point and a second input point connected to said balanced input terminal, said lattice structured resonator group transducing the input signal in the balanced state added from the first input point and the second input point into a surface acoustic wave and an electric signal and outputting the surface electric wave and the electric signal to said unbalanced output terminal, a first resonator disposed between the first input point and said balanced input terminal, and a second resonator disposed between the second input point and said balanced input terminal.
A twentieth aspect of the present invention is a communication apparatus, comprising an FM modulator for FM-modulating an audio signal added from a microphone, a mixer for mixing the FM-modulated signal put out from said FM modulator and a local oscillation signal added from a PLL oscillator, a band pass filter for filtering a signal put out from said mixer, a transmission amplifier for amplifying a signal filtered by said band pass filter, and an antenna for transmitting a radio frequency wave that is a signal amplified by said transmission amplifier, wherein said band pass filter has a balanced input terminal to which a signal mixed by said mixer is added in a balanced state, a balanced output terminal from which a signal is put out in a balanced state, a lattice structured resonator group, connected between said balanced input terminal and said balanced output terminal, said lattice structured resonator group having a first output point and a second output point connected to said balanced output terminal, said lattice structured resonator group transducing the input signal in the balanced state added from the first input point and the second input point into a surface acoustic wave and an electric signal and outputting the surface electric wave and the electric signal to the first output point and the second output point, a first resonator disposed between the first output point and said balanced output terminal, and a second resonator disposed between the second output point and said balanced output terminal.
A twenty-first aspect of the present invention is a communication apparatus, comprising a reception amplifier for amplifying a signal received by an antenna, a band pass filter for extracting a desired band signal from the amplified signal amplified by said reception amplifier, a mixer for mixing the desired band signal extracted by said band pass filter and a local oscillation signal added from a PLL oscillator through a local oscillation filter, an IF filter for filtering an intermediate frequency of the signal mixed by said mixer; and an FM demodulator for demodulating a signal filtered by said IF filter, wherein said local oscillation filter has an unbalanced input terminal to which the local oscillation signal oscillated by the PLL oscillator is added in an unbalanced state, an input transducer unit for exciting a surface acoustic wave with the local oscillation signal added from said unbalanced input terminal, an output transducer unit for receiving the surface acoustic wave excited by said input transducer unit and obtaining a desired signal, a first balanced output terminal and a second balanced output terminal from which a desired signal obtained by said output transducer unit is put out to said mixer in a balanced state, a first resonator disposed between said first balanced output terminal and said output transducer unit, and a second resonator disposed between said second balanced output terminal and said output transducer unit.
A twenty-second aspect of the present invention is a communication apparatus, comprising a reception amplifier for amplifying a signal received by an antenna, a band pass filter for extracting a desired band signal from the amplified signal amplified by said reception amplifier, a mixer for mixing the desired band signal extracted by said band pass filter and a local oscillation signal added from a PLL oscillator through a local oscillation filter, an IF filter for filtering an intermediate frequency of the signal mixed by said mixer, and an FM demodulator for demodulating a signal filtered by said IF filter, wherein said local oscillation filter has a balanced input terminal to which the local oscillation signal oscillated by the PLL oscillator is added in a balanced state, an input transducer unit for exciting a surface acoustic wave with the local oscillation signal added from said balanced input terminal, an output transducer unit for receiving the surface acoustic wave excited by said input transducer unit and obtaining a desired signal, a first balanced output terminal and a second balanced output terminal from which the desired signal obtained by said output transducer unit is put out to said mixer in a balanced state, a first resonator disposed between said first balanced output terminal and said output transducer unit, and a second resonator disposed between said second balanced output terminal and said output transducer unit.
A twenty-third aspect of the present invention is a communication apparatus, comprising a reception amplifier for amplifying a signal received by an antenna, a band pass filter for extracting a desired band signal from the amplified signal amplified by said reception amplifier, a mixer for mixing the desired band signal extracted by said band pass filter and a local oscillation signal added from a PLL oscillator through a local oscillation filter, an IF filter for filtering an intermediate frequency of the signal mixed by said mixer, and an FM demodulator for demodulating a signal filtered by said IF filter, wherein said local oscillation filter has an unbalanced input terminal to which the local oscillation signal oscillated by the PLL oscillator is added in an unbalanced state, a balanced output terminal from which a signal is put out in a balanced state, a lattice structured resonator group, connected between said unbalanced input terminal and said balanced output terminal, said lattice structured resonator group having a first output point and a second output point connected to said balanced output terminal, said lattice structured resonator group transducing the input signal in the unbalanced state added from said unbalanced input terminal into a surface acoustic wave and an electric signal, filtering the surface electric wave and the electric signal as the local oscillation signal in a balanced state, and outputting the local oscillation signal to the first output point and the second output point, a first resonator disposed between the first output point and said balanced output terminal, and a second resonator disposed between the second output point and said balanced output terminal.
A twenty-fourth aspect of the present invention is a communication apparatus, comprising a reception amplifier for amplifying a signal received by an antenna, a band pass filter for extracting a desired band signal from the amplified signal amplified by said reception amplifier, a mixer for mixing the desired band signal extracted by said band pass filter and a local oscillation signal added from a PLL oscillator through a local oscillation filter, an IF filter for filtering an intermediate frequency of the signal mixed by said mixer, and an FM demodulator for demodulating a signal filtered by said IF filter, wherein said local oscillation filter has a balanced input terminal to which the local oscillation signal oscillated by the PLL oscillator is added in a balanced state, a balanced output terminal from which a signal is put out in a balanced state, a lattice structured resonator group, connected between said balanced input terminal and said balanced output terminal, said lattice structured resonator group having a first output point and a second output point connected to said balanced output terminal, said lattice structured resonator group transducing the input signal in the balanced state added from said balanced input terminal into a surface acoustic wave and an electric signal, filtering the surface electric wave and the electric signal, and outputting the surface electric wave and the electric signal to the first output point and the second output point, a first resonator disposed between the first output point and said balanced output terminal, and a second resonator disposed between the second output point and said balanced output terminal.
According to the first and third aspect of the present invention, a first resonator is disposed between a first balanced output terminal and an output transducer unit. In addition, a second resonator is disposed between a second balanced output terminal and the output transducer unit. Thus, when an overvoltage or the like is applied to the first and second balanced output terminal in the inverse direction of a normal signal, the overvoltage is blocked by the first and second resonators. Thus, the overvoltage does not almost affect a circuit disposed upstream of the resonators (namely, the output transducer unit).
According to a second aspect of the present invention, a first resonator is disposed between a first balanced input terminal and an input transducer unit. In addition, a second resonator is disposed between a second balanced input terminal and the input transducer unit. Thus, when an overvoltage that is much higher than a normal signal is applied to the first and second balanced input terminals, the overvoltage is blocked by the first and second resonators. Thus, the overvoltage does not almost affect a circuit disposed downstream of the resonators (namely, the input transducer unit).
According to a fourth and sixth aspect of the present invention, a first resonator is disposed between a first output point and a balanced output terminal. In addition, a second resonator is disposed between a second output point and a balanced output terminal. Thus, when an overvoltage is applied to the balanced output terminal in the inverse direction of a normal signal, the overvoltage is blocked by the first and second resonators. Thus, the overvoltage does not almost affect a circuit disposed upstream of the resonators (namely, an output transducer unit).
According to a fifth aspect of the present invention, a first resonator is disposed between a first input point and a balanced input terminal. In addition, a second resonator is disposed between a second input point and the balanced input terminal. Thus, when an overvoltage or the like that is much higher than a normal signal is applied to first and second balanced input terminals, the overvoltage is blocked by the first and second resonators, the overvoltage does not almost affect a circuit disposed downstream of the resonators (namely, a lattice structure resonator group).
According to seventh to twelfth aspects of the present invention, when an overvoltage or the like is applied from a second signal outputting means to a surface acoustic wave device in the inverse direction of a normal signal, the overvoltage is blocked by the surface acoustic wave device. Thus, a first signal outputting means disposed upstream of the surface acoustic wave device can be protected against the overvoltage. In addition, when an overvoltage or the like that is much higher than a normal signal is added from the first signal outputting means to the surface acoustic wave device, the overvoltage is blocked by the surface acoustic wave device. Thus, the second signal outputting means disposed downstream of the surface acoustic wave device can be protected against the overvoltage.
According to thirteenth to sixteenth aspect of the present invention, when an overvoltage or the like is applied from a mixer to a band pass filter in the inverse direction of a normal signal, the overvoltage is blocked by the band pass filter. Thus, a reception amplifier disposed upstream of the band pass filter can be protected against the overvoltage.
According to seventeenth to twentieth aspects of the present invention, when an overvoltage or the like is applied from a transmission amplifier to a band pass filter in the inverse direction of a normal signal, the overvoltage is blocked by the band pass filter. A mixer disposed upstream of the band pass filter can be protected against the overvoltage.
According to twenty-first to twenty-fourth aspects, when an overvoltage is applied from a mixer to a local oscillation filter in the inverse direction of a normal signal, the overvoltage is blocked by the local oscillation filter. Thus, a PLL oscillator disposed upstream of the local oscillation filter can be protected against the overvoltage.
According to twenty-fifth to thirtieth aspects of the present invention, since the structure of a first resonator is almost the same as the structure of a second resonator, a balanced output signal can be obtained.
According to thirty-first to thirty-third aspects of the present invention, when a first resonator and a second resonator are adjusted so that the following factors are satisfied:
0.85xe2x89xa6(L1xc3x97n1)/(L2xc3x97n2)xe2x89xa61.15
wherein n1 is the number of comb line electrodes of the first resonator; n2 is the number of comb line electrodes of the second resonator; L1 is the aperture length of the first resonator; and L2 is the aperture length of the second resonator, unbalanced factors can be compensated. Thus, a signal that satisfies better unbalanced output conditions or better balanced input conditions can be obtained.
According to a thirty-fourth aspect, a third resonator is disposed between an unbalanced input terminal and an input transducer unit along with a first resonator and a second resonator. Thus, when an overvoltage that is much higher than a normal signal is added, the overvoltage is blocked by the third resonator. Consequently, the overvoltage does not almost affect a circuit disposed downstream of the third resonator (namely, the input transducer unit). With the effects of the first resonator and the second resonator, both the input and output sides can be protected against the overvoltage.
According to a thirty-fifth aspect of the present invention, a third resonator is disposed between an unbalanced output terminal and an output transducer unit along with a first resonator and a second resonator. Thus, when an overvoltage or the like is applied in the inverse direction of a normal signal, the overvoltage is blocked by the third resonator. Thus, the overvoltage does not almost affect a circuit disposed upstream of the third resonator (namely, the output transducer unit). Consequently, with the effects of the first resonator and the second resonator, both the input and output sides can be protected against the overvoltage.
These and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of a best mode embodiment thereof, as illustrated in the accompanying drawings.
This application claims priority on Japanese Patent Application No. 10-055562 filed on Mar. 6, 1998, the contents of which are incorporated herein by reference.